Why am I on a roof?

It's because there's a race going on. Normally, when you join the server you would be just barely inside Los Santos International Airport, but because people are having fun and you aren't currently involved you get to stay in timeout for a while. There's no escape, just hold Z and look at the player list! You can see times and a lap count, but sadly no indication of how long the race is going to last.

Just wait it out, eventually they will stop being in the race and you will be able to join them! (… once we go over the controls, which is a little later.